The present invention relates to an X-ray inspection apparatus (typically an X-ray CT scanner) in which errors due to variations of an X-ray focal point are properly compensated for, thereby improving an accuracy of the result of inspection.
An X-ray source is generally subjected to periodical variations due to ripples of the acceleration voltage of an X-ray tube, which is caused by the existence of a peripheral cap of the X-ray tube filament, and periodical variations due to electrical field variations.
The former ripple variations can be detected as variations in a total X-ray radiation amount per unit time. Further, a recent rectifier circuit for generating the acceleration voltage is highly improved such that the amount of the ripples is at most 1% of the acceleration voltage. Consequently, the ripple variations are no longer serious for a recent technology.
On the other hand, when the X-ray tube filament is powered by an AC current, an electric field for converging hot electrons of the X-ray tube becomes alternative, so that the latter electrical field variations occur. The electrical field variations cause variations in a focal spot size at the X-ray target, thereby producing moires in the reconstructed image of an inspection object. The focal spot size variations do not change the total amount of X-ray radiations per unit time, but they change the X-ray radiation amount per unit area.
An error compensation for X-ray inspection data should be done in consideration not only of the ripple variations, but also of the electrical field variations. However, according to a prior art technology, the electrical field variations can hardly be detected.
Generally, an X-ray inspection apparatus (X-ray CT scanner) is used for a diagnostic purpose and industrial purpose. Recent demand in the industrial purpose is to obtain an extremely high resolution of the order of several microns to several tens microns. Obtaining such a high resolution is new trial for a prior art X-ray CT scanner, and it requires a special microfocus X-ray generator. The focal spot size of 5 .mu.m to 100 .mu.m, obtained by such a special microfocus X-ray generator, is roughly 1/100 of the focal spot size of a conventional X-ray generator.
When the X-ray CT scanner operates for a long time to acquire a large amount of inspection data, the position of the microfocal spot is liable to drift with passage of time and/or a change of temperature. If a position drift of the microfocal spot occurs, artifacts appear in the image reconstructed from the acquired inspection data.
Thus, for an extremely high resolution X-ray CT scanner, variations in the microfocal spot size due to electrical field variations, as well as variations in the position of the microfocal spot due to drift, are serious problems.